Maybe I am the lone person here but I think they are absolutely worthless junk. They are not worth the money. They gain what, 2hp on the wheels? At the cost of slowing you acc. Save your money. You will see more gain by eliminating the clutch fan like mentioned above.
An Underdrive pulley refers to an aftermarket crankshaft or accessory pulley (such as an alternator pulley) that is designed to drive a vehicle's accessories at a slower rate than stock. Originally they were invented for race cars in the 1950s and 60s, whose engines had been modified and ran at higher RPMs for longer periods of time than the manufacturers had intended. It was therefore necessary to underdrive the belt-driven accessories (among other things like the water pump) to keep the accessories from burning out and the engines running reliably.
Presently, these pulleys are marketed as performance enhancers that actually produce additional torque and horsepower by reducing parasitic drag caused by belt-driven accessories, or by being lighter and reducing rotational mass, or both. However, the gain is usually minimal, on the order of 1-6 wheel horsepower depending on the engine. There are trade-offs for this "free power," however.
Changing the original crankshaft pulley can have negative effects on the harmonics of the crankshaft: a pulley that is too light or not properly weighted cannot reliably balance natural frequencies that occur along the length of the crankshaft at certain RPMs. The original pulley may have been carefully designed, balanced and weighted to counteract (harmonically dampen) this naturally occurring harmonic frequency, and without it, eventual engine damage may result. See: Engine Balancing
A less severe side effect is somewhat intended by the design of these pulleys. Since the alternator, power steering, and/or air conditioning units are not spinning as fast, corresponding dips in alternator voltage, power assist, and A/C effectiveness may be noticeable, especially at idle. The headlights may dim when coming to a stop, or the stereo may lower in volume, for instance. For a race car, this is not of concern, but for a daily driven vehicle it can be an annoyance.